The present invention relates to watthour meter registers and, more particularly, to apparatus for changing a rate per unit consumption based on seasonal or other criteria.
Utilities generally consider that the cost of electric energy can be attributed to two factors: (1) the out-of-pocket cost for generating the energy (fuel, operating and maintenance personnel and equipment, etc.) and (2) the capital cost of the generating equipment must be installed to generate the electric energy.
Electric energy consumption is far from constant over a day, month or year. In some seasons such as, for example, the air-conditioning season, power demand is much greater than at other seasons. In order to provide adequate service, the utility must provide generating equipment capable of satisfying the maximum demand which it may experience. Thus, the utility must bear the capital cost of installing sufficient generating capacity to satisfy the peak demand. It must do this while knowing that, except at peak times, a substantial portion of its generating capacity remains idle. According to this analysis, the capital component of the total energy cost is governed by the peak demand in the peak season.
Some rate-setting bodies have recognized the importance of peak demand and have established tariffs in a manner designed to encourage energy consumers to limit peaks in their energy usage and to shift energy consumption from peak to off-peak times. One of such tariffs permits the utility to measure the amount of energy consumed in a sequence of demand intervals. At the end of each demand interval, the demand in the just-completed demand interval is compared with a stored value representing the highest demand in any demand interval since the register was last reset. If the demand in the just-completed demand interval is higher than the previous high demand value, then the value of the demand in the just-completed demand interval is stored and the previously-stored value is deleted. The value of the maximum demand existing at the time the meter is read determines the rate the consumer must pay for all of its electric energy. Thus, the consumer receives a powerful economic incentive to limit the peak load placed on the utility system.
In most utility systems, the demand register is reset by a meter reader or by automatic remote reading equipment when the readings on the register are collected for billing purposes. In some systems, not of concern to the present invention, more than one high demand value is stored. For example, the five highest values during the demand interval may remain available for internal or external purposes.
A further economic device for limiting maximum demand accounts for the reality of peak consumption seasons. Rate-setting bodies permit utilities to charge higher rates during some seasons (air-conditioning season is one example) than during others. This technique is known as time-of-use metering.
A problem arises in selecting the manner, or time, for changing from one rate to another in time-of-use metering. One technique for selecting a time to change rates includes automatic change from one rate to the other at the beginning (midnight) of the first day of each rate season. An internal clock/calendar maintains an updated file containing the present time and date. The dates on which seasonal change is to occur is stored in a change-date library. At midnight each day, the present date is compared with the dates on which seasonal change is required. When a match is found, a rate change is performed. If such automatic rate change is made in a register which includes a demand register, when the register is read at a later time, there may be no indication of rate prevailing at the time the maximum demand was recorded. Thus, either the consumer may be unfairly penalized, or the utility may lose revenue to which it is entitled.